Acquisition of seismic data at the sea bottom is a methodology affording many advantages and great possibilities of obtaining multicomponent seismic data of very high quality at a reasonable cost.
The main goals of marine seismic surveys in deep and ultradeep offshore zones are:                finer characterization of reservoirs by acquisition of the complete wave field (4C) allowing to reduce drilling risks, and        reservoir monitoring to detect variations in a reservoir by repeated seismic acquisitions (4D).        
The location of seismic receivers on the sea bottom allows to eliminate the multiples due to the water layer and, since the ambient noise is lower, to increase the signal amplitude, which leads to an improvement in the seismic images. Recording the S waves and the P waves allows to obtain additional structural or mechanical information concerning the lithology, the fluids, the fractures, and also facilitates imagery of complex structures and in the presence of gas.
The goal of repetitive seismic surveys is to detect variations in underground reservoirs due to the production of hydrocarbons. It is therefore necessary to carry out repeated and staggered seismic measurements. The differences between two measurements represent the variations that have occurred in the reservoir. The measuring accuracy and the repetitiveness must be guaranteed. The measuring conditions therefore have to be strictly identical between two surveys, which are generally staggered over a period of several months. The key parameters are the position, the orientation and the coupling of the pickups. The amplitude of the measurements, inherent in the errors linked with these parameters and the noise due to the instruments, has to be significantly lower than the signal reflecting the changes in the reservoir. The existing 4D multicomponent data sets do not allow quantification of the differences with precision.
Activity in the field of seismic exploration of underground zones lying under a deep water layer ranging between about 300 m and 1500 m (referred to as deep offshore) and even deeper (ultradeep offshore) is in full expansion. The average size of the oil fields that are going to be developed in future increases considerably with the water depth Since the start-up costs, the production costs and the associated risks are all the higher as the depth of immersion increases, it is all the more important to optimize production. This requires new exploration and production technologies allowing to reduce uncertainties and to produce more rapidly at an acceptable cost.
Seismic methods are developing constantly. However, despite improved results, their cost is still considered to be too high by oilmen to be applied systematically. Multicomponent seismic prospecting (3C/4C) and seismic prospecting referred to as repetitive (4D) are clearly identified currently as technologies with the highest expansion potential and which will join together to such an extent that it will be possible to manage underground reservoirs by using the additional data available and to combine them with the data obtained from the instruments in the well. The challenge consists in putting on the market a seismic exploration device with pickups located on the sea bottom at a reasonable cost, while taking account of the profit provided by such systems in terms of information abundance and logistic advantages. The most suitable exploration device must meet the requirements of seismic prospecting for reservoir characterization as well as reservoir monitoring during development, notably under deep offshore conditions, while being reliable, effective and economical.
OBC type seismic prospecting methods using a seismic cable or streamer laid on the sea bottom are well-known. Seismic receivers such as geophones mounted on U-joints to provide correct orientation or hydrophones are arranged all along this part and are coupled with the bottom The receivers are connected by lines internal to the streamer to an acquisition equipment on a boat at the surface. Several cables can be laid in parallel, thus forming a 3D seismic device. Such streamers are commonly used down to depths of about 500 m and even, in some cases, of more than 1000 m The streamers are towed immersed within the context of seismic prospecting operations or permanently installed within the context of long-term monitoring (4D seismic monitoring). A second boat is used to move the seismic source. Unless the cables are buried in the sediments, coupling of the pickups with the sea bottom is not optimal and, despite a large number of pickups, the seismic data acquired are of average quality.
Devices of this type are for example described in patents U.S. Pat. No. 4,870,625, WO-99/23,510, 98/07,050 or 97/13,167.
It is also well-known for oceanographic research, notably for surveys of the structure and the seismicity of the continental margin, to lower down to the sea bottom (OBS type) acquisition stations consisting each of a sealed box containing seismic receivers, hydrophones and geophones, etc., and the associated electronic equipment, allowing continuous recording of the low-frequency seismic signal and storage of the data in a mass memory. Coupling of the pickups in the sediments is satisfactory if the module comprising the geophones is outside the station. Recovery of the acquired data occurs after pulling the equipment from the bottom to the surface. A launching device activated by acoustic control from the boat at the surface allows disconnection of a ballast, then pulling of the acquisition equipment to the surface, and signaling devices such as a flag and a warning light allow to locate it at sea After each use, a station is reconditioned prior to a new immersion. Most of these stations work at depths that can reach 6000 m. The number of stations used for a scientific mission is relatively small and the distance between the stations can range from several hundred meters to several hundred kilometers. The recording time, which can range from one week to several months, is conditioned by the data storage capacity and by the range of the supply battery.
Devices of this type are for example described in patents U.S. Pat. No. 4,422,164; 4,692,906 or 5,189,642.
It is also well-known to install at the sea bottom seismic data acquisition units so as to improve coupling of the pickups with the underlying formation.
This installation can be carried out using a subsea robot or by launching from the surface acquisition units that are sufficiently streamlined to directly stick into the bottom under the effect of gravity. These seismic signal acquisition units include a streamlined part or boom provided with a housing for at least one seismic receiver (a 3C geophone and a hydrophone for example), inclinometers for measuring their orientation, a compass and a seismic data collection module, as well as means of positioning each acquisition unit at the bottom (acoustic telemetry), and surface means for recovery of the seismic data collected. Recovery of these units at the end of an operation is also performed by means of the robot. A device of this type is intended to work at water depths up to 1500 m.
Such acquisition devices are for example described in French patent application 00/16,536 or in patents FR-2,738,642 and U.S. Pat. No. 4,134,097.
Patent FR-2,774,775 filed by the applicant also describes a method intended for seismic exploration of an offshore reservoir wherein one or more seismic emission units comprising one or more seismic sources associated with power supply means are lowered to the sea bottom, each one of these units being connected to the surface by a multifunction umbilical. Seismic receivers can also be coupled with the surface from the bottom.
The underwater acquisition devices, whether mobile or coupled with the bottom of the water mass, are generally associated with acoustic positioning means. The relative position of the devices in relation to several surface buoys provided with satellite locating means (GPS positioning system) is located by acoustic telemetry. Locating devices combining acoustic telemetry and satellite positioning are for example described in patents U.S. Pat. No. 5,119,341 and 5,579,285.
Patent application FR-2001/16,652 filed by the applicant describes another multicomponent seismic data acquisition device that can be used within the context of active seismic prospecting surveys or for long-term detection of microseismic events generated by natural evolutions in the subsoil (passive monitoring). It comprises a central control and acquisition station, a plurality of seismic bottom acquisition stations suited to go down, under the effect of gravity, to the bottom of the water mass and to enter it so as to couple the seismic receivers with the underground formation, and several relay buoys suited to float at the surface of the water and comprising each for example GPS type satellite positioning means, and radio and acoustic wave transmission means allowing to position the bottom stations and to relay the exchanges between them and the central control and recording station.